As known, in electronic devices, the term "noise" indicates a random fluctuation in currents or voltages at the device terminals, and may seriously limit the minimum signal level that can be handled by the device.
The noise in each device is due to various physical causes, some of which have been known for some time. Of particular interest are what are known as "flicker" noise (also indicated 1/f) and "burst" noise, the first of which exists in all and the second in a significant percentage of devices.
Flicker noise is commonly acknowledged to be caused by fluctuations in the number of carriers, due to entrapment of the carriers in surface layers of the device, i.e., due to tunneling at the semiconductor-oxide interface. According to accepted theory, the carriers in the semiconductor may communicate with trap levels at a given distance within the tunnel oxide layer, and remain trapped for some time prior to being re-emitted. In the case of transistors, in particular, flicker noise sources are located at the base-emitter junction.
Flicker noise is especially undesirable in the case of operational amplifier input transistors and audio preamplifiers.
Burst noise, on the other hand, is caused by a sharp variation in current between two or more constant values. Variation frequency may be very low (less than 1 Hz) or high (hundreds of hertz), in which case, burst noise may be confused with a high degree of flicker noise. This type of noise is generally attributed to the presence of defects, metal inclusions and precipitates in the space charge region of the junction; and the fluctuation in current depends on the extent, if any, to which the defect participates in conduction. The fact that burst noise is reduced by deficiency-reducing processes, such as gettering, would appear to bear out this theory.
As is known, for reducing flicker and burst noise it is necessary, for a given collector current I.sub.C, to reduce base current I.sub.B and intrinsic base resistance r.sub.bb ' (below emitter), so as to reduce input noise equivalent current I.sub.n (which is proportional to the square root of base current I.sub.B) and noise equivalent voltage e.sub.n (which is proportional to intrinsic base resistance). Solutions have been thus studied for reducing the base current (then increasing transistor gain) and the intrinsic base resistance. Such solutions however present low breakdown voltages, in particular a low collector-to-emitter, short-circuited base, breakdown voltage BV.sub.CES (measured with the emitter short-circuited to the base) due to punch-through.